A single teaspoon of living soil can contain billions of bacteria, protozoa and fungi. When those soil microbes do what they do best—feed, reproduce and live—they produce mucilage or biofilm, which are nice words for slime.

That slime is absolutely necessary to build good soil structure and maintain living soil.

“A slimelike mucilage is secreted by microbes when they break down organic matter. This substance cements tiny soil particles into larger ones,” said David Trinklein, horticulture specialist for University of Missouri Extension. “The result is porous soil, which is important for good root growth.”

That porous soil serves another important purpose. It lets air reach the root zone.

“We forget that there is an atmosphere below the surface of the ground,” Trinklein said. “And we forget the fact that a plant’s root system, just as its shoot system, needs oxygen.”

Along the way, the decomposition process creates a highly complex organic substance called humus.

“‘Humus’ is a word that should be emblazoned upon every gardener’s mind,” Trinklein said. “‘Hummus’ is what we eat. Humus—one ‘m’—is a gardener’s best friend.”

Humus is the dark material that forms when organic matter decays. It feels spongy when wet but loose and crumbly when dry. Humus helps soil drainage even though it can hold a lot of water. It’s also a good source of plant nutrients.

The addition of organic materials is required to turn soil into humus. There are many sources of organic material. Peat humus is very well-decomposed peat moss. It’s an excellent soil amendment but it can be a bit pricey, Trinklein said. Or you can add compost, provided that it’s herbicide-free. This would be the least expensive soil amendment because it can be made from collected grass clippings and leaves from your yard.

“One good management practice is to incorporate into garden soil about 4 inches of well-decomposed organic matter, to a depth of 8-12 inches, each and every time you work the soil in the spring,” he said.

You’ll know the soil structure is in good shape when it can pass the water drainage test:

Dig a hole 12 inches deep, fill it with water and let it drain overnight. Then fill it again with water. The first filling saturates the soil; the second filling measures how fast the water drains.

“Ideally, soil should drain at the rate of about 2 inches per hour. Therefore, it should take six hours for the hole to drain the second time,” Trinklein said.

Once you amend for soil structure, it’s time to turn to soil fertility. This requires a soil test. The soil needs adequate amounts of certain essential mineral elements for plants to flourish. Nitrogen, phosphorus, potassium, calcium, magnesium and sulfur are all macroelements that are needed in fairly large quantities.

“A soil test will give us the current nutritional status and provide recommendations on how to improve it,” Trinklein said.

A soil test will also provide information on the soil’s pH. This is a measurement of the acidity or basicity of the soil. The test will indicate if you need to add lime or sulfur to adjust the pH. Most garden plants grow best within a pH range 6.2 to 6.8.

By turning your attention to the overall health of the soil, you’ll increase the odds of enjoying gardening success, Trinklein said. Adding organic matter year after year eventually results in what gardeners call “black gold”—a dark, living soil that holds moisture and nutrients but still drains well.

The following MU Extension publications are available for free download: